1. Field of the Invention
The present invention relates to a catalyst using a precious metal. More particularly, the present invention relates to a highly efficient catalyst using a three-dimensional porous structure as a supporting material for the precious metal in order to prevent an inaccessible region of the precious metal from being generated, and further improve the diffusion of exhaust gas.
2. Description of Related Art
Recently, according to the increasing usage of vehicles and severe traffic, air pollution by exhaust gas is becoming an issue. In order to regulate exhaust gas and enforce the regulations, many countries have established emission standards for pollution substances such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) in exhaust gas.
Usually, catalysts coated with precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) are used for removing noxious substances from the exhaust gas of vehicles. These catalysts remove the noxious substances from the exhaust gas and purify the exhaust gas by promoting decomposition of the hydrocarbons, oxidization of the carbon monoxide, and reduction of the nitrogen oxide.
A catalyst uniformly coated with a precious metal in a supporting material as a purifying catalyst of exhaust gas has been published. In a case of a conventional catalyst for purifying vehicle exhaust gas, the catalyst is manufactured by coating catalytic substances including the expensive precious metal on a supporting material shaped as a honeycomb in order to increase the contact area between the catalyst and the exhaust gas, and thus to increase the reaction area.
Typically, square cells are used as the unit cells of the honeycomb supporting material. However, as shown in FIG. 1, using square cells in a catalyst creates corners where certain amount of catalytic substances accumulates resulting in a thicker catalytic layer 14 in the vicinity of the corners.
Noxious reactants 16 such as CO, HC, and NOx in the exhaust gas diffuse into the catalytic layer and are then converted into harmless substances in contact with the precious metal (Pt, Pd, and Rh). The arrow in FIG. 1 shows the diffusion of the exhaust gas around a corner. Because the catalytic layer there is thicker, the exhaust gas cannot diffuse into the region 15 of the precious metal. As such, the catalyst in deep corners becomes an inaccessible region, that is, a dead zone into which the CO, HC, and NOx cannot diffuse. So the precious metal in the inaccessible region 15 cannot participate in reaction.
To solve the above problems, a hexagonal cell 20 in FIG. 2 was contrived. But the isotropic strength of the hexagonal cell 20 is weaker than that of the square cell 10, so few hexagonal cells 20 are actually used.
FIG. 3(A) shows a schematic view of a diffusion path of reactants in a unit cell where 12 refers to a cell wall and can be made of cordierite. FIG. 3(B) and FIG. 3(C) show schematic views of two catalysts where the one shown in FIG. 3(C) is deteriorate. In the past, an amorphous powder was used as a supporting body 50, so the density of the catalytic layer grew large, the diffusion of the exhaust gas was deteriorated thereby, and an inaccessible region of the precious metal 15 was generated. In addition, the reaction surface was reduced and pores were covered after the precious metal was sintered (referring to 40 and 50a) or after the precious metal was reacted with sulfur in exhaust gas and was poisoned or otherwise contaminated (referring to 55). Furthermore, inaccessible regions of precious metal 50b were generated.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.